Method and device for locating magnetic implant source field

ABSTRACT

An apparatus and method for locating a magnetic implant in a surgical application using the field of a source magnet for the implant guiding field. The source magnet is an electromagnet having a separate calibrated magnetic field component in addition to the guiding field, so that both the magnitude and orientation of the magnetic field as a function of position around the magnet are known. A magnetic implant is provided with a sensor, such as a three-axis Hall effect sensor, to provide an indication of the magnitude and orientation of an applied magnetic field when the implant is surgically implanted in a patient. After implantation, the source magnet is energized with a current having a modulated component. The modulated component is received and filtered from the signal received from the Hall effect sensor in the implant, and provided to a processor that computes the location of the implant relative to the electromagnet based upon the detected magnitude and orientation of the modulated component of the magnetic field, and the location and orientation of the electromagnet. Localizers may be used to supply the relative locations of the patient and the electromagnet to the processor. A display may be provided to display a representation of the location of the implant in the patient, which may also be superimposed over a preoperative image of the patient.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to methods for locating a magnetic implant, andmore specifically, to a method for locating a magnetic object beingguided, in a surgical application, by the field of a source magnet.

(2) Description of Related Art

In the filed of surgery, there exists a need to control the orientation,forces, and/or motion of internally implanted devices. One method thathas been used to control such implanted devices is the application of amagnetic field from an external magnet. In this method, the magneticfield acts upon the implanted device, which itself comprises magneticmaterial, which may be in the form of a permanent magnet. In accordancewith prior art practice, a physician surgically implants the devicecomprising magnetic material and then guides the position of theimplanted device by moving an external permanent magnet and observingthe resultant movement directly with an X-ray fluoroscope. Examples ofthe prior art may be found in a review article by Gillies et al.,“Magnetic Manipulation Instrumentation for Medical Physics Research,”Rev. Sci. Instrum. 65, 533 (1994). See also McNeil et al., “FunctionalDesign Features and Initial Performance Characteristics of aMagnetic-Implant Guidance System for Stereotactic Neurosurgery,” IEEETrans. Biomed. Engrg., 42, 793 (1995); Tillander, “Magnetic Guidance ofa Catheter with Articulated Steel Tip,” Acta Radiologa 35. 62 (1951);

Frei et al, “The POD (Para-Operational Device) and its Applications,”Med. Res. Eng. 5, 11 (1966); U.S. Pat. No. 3,358,676 to Frei et al.,issued Dec. 19, 1967, entitled “Magnetic Propulsion of Diagnostic orTherapeutic Elements Through the Body Ducts of Animal or HumanPatients”; Hilal et al., “Magnetically Guided Devices for VascularExploration and Treatment,” Radiology 113, 529 (1974); Yodh, et al., “ANew Magnet System for Intravascular Navigation,” Med. & Biol. Engrg., 6,143 (1968); Montgomery et al., “Superconducting Magnet System forIntravascular Navigation,” Jour. Appl. Phys. 40, 2129 (1969); U.S. Pat.No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled “MagneticallyGuidable Catheter-Tip and Method”; and U.S. Pat. No. 3,794,041 to Freiet al., issued Feb. 26, 1974, entitled “Gastrointestinal Catheter.” Thefull content of each of the cited documents are herein incorporated byreference in their entirety.

Obviously, the above-described technique requires the physician to reactto the movement of the implanted device. Determination of this movementcan be a problem, because the implanted device can, in general, move inthree-dimensional space inside the patient. With prior art hand-heldmagnets, the only feedback the surgeon could have was his observation ofmotion of a magnetic implant by x-ray or ultrasonic imaging in responseto his movement of the magnet. Usually, fluoroscopic imaging isemployed. However, fluoroscopic imaging can be subject to interferencefrom the magnet, itself. In difficult interference situations, it isdifficult without proper imaging guidance to provide even a reasonableguess as to a correct direction for the magnet axis to obtain fieldalignment with the intended path. The large electromagnet of Yodh et al.(supra) is one attempt to minimize the “blindness” of the approach justdescribed, but the Yodh et al. approach still relies on operatorjudgment and vision, and is subject to such error. While multiple coilarrangements such as the magnetic stereotaxis system (MSS) described inMcNeil et al. (supra) can be used to provide such guidance, it isdifficult in such systems to provide a combined guiding force andforce-applying field gradient in the same desired direction.

U.S. Pat. No. 5,558,091 issued Sep. 24, 1996 to Acker et al., which ishereby incorporated by reference in its entirety, discloses a magneticposition and orientation determining system using magnetic fields. Bymonitoring field components detected at a probe during application ofthe fields, the position and orientation of the probe in the field canbe determined. A representation of the probe can be superimposed on aseparately acquired image of the subject to show the position andorientation of the probe with respect to the subject. Although thedevices and methods disclosed in this patent can determine the locationof an implant, the magnetic fields used are so small as to not exert anysignificant, perceptible forces on magnetic materials in the sensingregion. There is no disclosure or suggestion to use a magnetic field toboth align and/or guide an implanted probe as well as determine itslocation via an externally applied magnetic field, nor is there anysuggestion to place strongly magnetizable materials or permanent magnetsin a seed or on a probe, adjacent to a magnetic sensor, in such a manneras to allow accurate determination of location using the magnetic sensorin the immediate vicinity of the seed.

Clearly, both operation time and risk to a patient could be reduced ifan apparatus and method were available to more accurately and reliablylocate, as well as guide, orient, and/or move a magnetic surgicalimplant. (For present purposes, when reference is made to “guiding” animplant, it should be assumed that this may also refer to “orienting” animplant, as well.) Preferably, while such apparatuses and methods mayallow the use of x-ray, ultrasonic, or fluoroscopic imaging devices,they should not require such imaging devices to provide the location ofthe implant. In addition, it would be advantageous if the locationmethod would not require the addition of magnetic field creatingdevices, such as is required by the Acker et al. patent, and which mightfurther increase the interference with the location and operation of aguiding magnet. If the location can be obtained without the use of suchadditional imaging and/or locating devices, the external magnet orelectromagnet (or magnets or electromagnets) used for guiding themagnetic implant may be provided with a larger, unobstructed range ofmotion. It would also be advantageous if the location can be obtainedwithout being subject to interference from the magnet itself, as occursor can occur with many common fluoroscopic imaging systems.

SUMMARY OF THE INVENTION

There is thus provided, in accordance with a first aspect of theinvention, a magnetic surgical implant comprising a flexible probehaving a magnetic seed mounted at a distal end thereof and a magneticfield sensor mounted near the magnetic seed and in a fixed relationshipthereto so that as an external magnetic field acts on the magnetic seedto guide or propel it through a patient's body, the magnetic fieldsensor is also acted upon and provides an output that can be correlatedwith the position of the magnetic seed.

This embodiment of the invention may include a '-axis magnetometer probeas the magnetic field sensor, which may be mounted within the flexibleprobe. Signals from the sensor may be conducted from the magnetic fieldsensor through the flexible probe by at least one sensor conductor. Theprobe itself may be a catheter or an endoscope.

There is also provided, in accordance with another aspect of theinvention, a device for providing position data for a magnetic surgicalimplant having a magnetic field sensor which is guided or propelledthrough a patient's body by an external magnet, said device comprising acurrent source for supplying a modulated electrical current to theexternal magnet to thereby generate an oscillating magnetic fieldcomponent, a demodulator coupled to the magnetic field sensor andresponsive to a magnetic field direction and magnitude of theoscillating magnetic field component to provide a signal indicative ofthe oscillating magnetic field component at the location of the magneticfield sensor, a memory containing a representation of a relationship ofspatial locations to a magnetic field pattern produced by the externalmagnet, and a data processor coupled to the demodulator and the memoryand configured to compute a spatial location of the magnetic fieldsensor as a function of the signal indicative of the oscillatingmagnetic field component.

In accordance with yet another aspect of the invention, a method forlocating a magnetic implant is provided, comprising the steps of (a)surgically implanting a magnetic implant including an associatedmagnetic probe in a patient; (b) applying a modulated magnetic fieldfrom an external electromagnet; (c) detecting signals from the magneticprobe resulting from the modulated magnetic field; and (d) calculatingthe relative location and orientation of the magnetic probe, andtherefore, the magnetic implant, with respect to the electromagnet fromthe detected signals from the magnetic probe.

Additional steps to the above-described method may be added, inaccordance with the invention. Such steps may include locating thepatient and the electromagnet so that an indication of the magneticprobe, and therefore, the magnetic implant, is displayed. An additionalstep of superimposing this indication over a preoperative image of thepatient may also be included.

It is thus an object of this invention to provide a method and apparatusfor locating a magnetic object which is being guided by the field of asource magnet, using the field of the source magnet itself to locate theobject.

It is a further object of this invention to provide a method andapparatus for accurately and reliably locating a magnetic surgicalimplant without requiring x-ray, ultrasonic, or fluoroscopic imagingdevices.

It is another object of this invention to provide a method and apparatusfor accurately and reliably locating a magnetic surgical implant byusing a magnet that simultaneously serves the functions of guiding amagnetic implant and locating it.

It is yet a further object of the invention to provide a method andapparatus for locating a magnetic surgical implant that permits a widerange of unobstructed motion to be provided to an external magnet orelectromagnet for guiding the magnetic surgical implant.

It is a still further object of the invention to provide a method andapparatus for locating a magnetic surgical implant that is not subjectto interference caused by the presence of the external magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the magnetic probe of the presentinvention; and

FIG. 2 is a perspective view of the field generating coil, along withequipment needed to use it for the location of the implant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating an embodiment 100 of an inventivedevice in accordance with a first aspect of the invention. This devicecomprises a flexible probe 102, which may be an endoscope or a cathetertube. A magnetic seed 104 of permanent magnetic, or at least a permeablematerial, is located near an end 106 of the flexible probe. Seed 104preferably comprises a samarium-cobalt (SmCo) permanent magnet, or evenmore preferably a neodymium-boron-iron (NdBFe) permanent magnet.Typically, seed 104 may be about 0.7 mm in diameter and length, but mayrange up to about 4-5 mm in diameter and up to 7-10 mm long, dependingupon the surgical application of probe 102. It is not intended toexclude seeds of other sizes, whether larger or smaller, from the scopeof the invention. Preferably, seeds 104 of the sizes given as exampleswould have magnetic fields in the immediate vicinity of seed 104 of nomore than 0.4 T. It is contemplated that the magnetic seed 104 isfixedly held in place in a suitable configuration inside or outside theprobe, so that when the end 106 of probe 102 is implanted in a patient,an externally-applied magnetic field can be used to guide, direct and/orpull end 106 of probe 102 through a desired path in the patient's bodyby means of magnetic forces applied to seed 104, so that medicaments ortherapy can be delivered to a selected location in the patient's body.

A magnetic field sensor 108, such as a 3-axis Hall effect probe, is alsoattached near the end 106 of probe 102, so that it is fixed in physicalrelationship to, and closely proximate magnetic seed 104 and the end ofprobe 102. Typically, the magnetic field from seed 104 at the locationof sensor 108 would be about half of that indicated above in theimmediate vicinity of the seed. Sensor or signal wires 110 from sensor108 are led out an end 112 of probe 102 opposite end 106. Sensor wires110 provide a signal or signals indicative of the magnitude andorientation of a magnetic field at or near the seed 104, and/or the end106 of probe 102. If probe 102 is an endoscope, an optical fiber (notshown) may be provided to pass through the probe to provide a view of aregion ahead of the endoscope. Other passages, not shown, may beprovided for other surgical purposes.

Referring now to FIG. 2, probe 102 is shown implanted in a patient 200resting on a table 202. A calibrated electromagnet 204 generating amagnetic field having a known magnitude as a function of current, andhaving a known orientation as a function of location relative toelectromagnet 204 is provided. Electromagnet 204 is used for locatingimplant 100, or more precisely, the field sensor 108 which is part ofimplant 100. Electromagnet 204 may also be used to generate forces orfield lines that may be used to move or to guide the movement of implant100 through patient 200. Electromagnet 204 may be attached to a moveablearm 206 for this purpose. Typically, electromagnet 204 is capable ofgenerating a field of between 0.1-0.3 T at the location of seed 104, orwhere it is intended that seed 104 is to be moved.

In a preferred configuration, a low frequency generator 208 provides asignal to a computer 210, which adds or mixes this signal with its owncomputation of the current required by electromagnet 204 to generate amagnetic field for purposes of guiding implant 100. This combined signalis sent to an amplifier/power supply 212, which provides current to coil204. Coil 204 generates magnetic field lines such as 214, 216, 218, 220,and 222 shown in FIG. 2. Sensor 108 of implant 100 lies on one of thesefield lines, shown as 222 in FIG. 2, and sensor 108 senses the strengthand direction of the field at the location of implant 100. Wires 110from the probe travel through the probe 102 (which is usually anendoscope or catheter), to instrument module 224, which converts thesignals to a magnetic field direction and magnitude as sensed by theprobe. This information is sent to a computer 226, which may be, butneed not be the same as computer 212. Computer 226 has also receivedinformation from mounting arm 228, which enables it to know the locationand orientation of magnet 204. As long as no rotation of the magnetabout its axis has occurred, there is no ambiguity of the field lines inquestion. For the front hemisphere of magnet 204, the field lines areknown to computer 226 (for example, by having a representation thereofin a reference frame of the magnet 204 stored in a memory 230), so thatthe computer 226 can match the data received from instrument module 224with data from the known field lines in memory 230, to determine thelocation and orientation of probe 106 relative to magnet 204. Thisinformation may be provided to a display 232, or it may be provided tocomputer 210 (or used by computer 210, if computers 226 and 210 are oneand the same computer) to provide a signal to the magnet power supply212 in a manner according to “Method and Apparatus Using Shaped Field ofRepositionable Magnet to Guide Implant,” U.S. Provisional Patent App.No. 60/065,107 to W. M. Blume et al., filed Nov. 12, 1997, and anon-provisional application by the same title as the provisionalapplication, also to W. M. Blume et al., and filed on even date with thepresent application (both the provisional and non-provisionalapplications referred to being herein incorporated by reference in theirentirety), or otherwise as may be known in the art.

The use of a modulated signal to generate the magnetic field of magnet204 allows various signal processing methods known to those skilled inthe art to achieve better accuracy in determining the location of probe102. With such modulation, for example, it becomes less necessary, orunnecessary, to account for a varying DC current to coil 204, whichwould otherwise have t taken into account by computer 226 in correlatingthe measured magnitude of the field with the stored representation ofthe relationship of spacial locations to the magnetic field patternproduced by electromagnet 204 that is stored in memory 230. Of course,the varying DC current could be taken into account by communicating thisinformation from computer 210 to computer 226, or if computers 210 and226 are one and the same computer.

It is thus seen that, in this invention, a low frequency oscillatingsignal is added—electronically summed—into the signal from a computerthat sets a current from the amplifier—power supplies to one or morecoils. The present invention is used with a single field generatingcoil, although accurately known fields from any coil assembly could beused if they do not have duplicate fields at more than one location. Theoscillating signal is detected by a very small 3-axis magnetic fieldsensor, for example a 3-axis Hall probe, attached to the magneticimplant. The signal wires for the probe are led out of the body by beingin the catheter or endoscope which contains or is attached to the guidedmagnetic implant, and which is being moved in the body duct.

The three Hall voltages from the magnetic field sensor provide adirection and magnitude for the oscillating magnetic field at thelocation of the probe. The oscillating signals are separated from thecontributions from the steady guiding field by the use of dc isolationor controlled biasing, and are made accurate by the well known techniqueof synchronous detection, or some other electronic noise reducing means.Since the desired guidance field is computer generated and the signal isthen sent to the magnet power supply—amplifier, this signal can easilybe used to insert an appropriate biasing voltage in each of the threesensor leads so as not to overload sensitive synchronous detection (orother) circuity for the locating function. The signal from the proximatemagnetic seed will either be a DC signal (if the seed is a permanentmagnet) that can be biased out, or an AC signal (if the seed is apermeable material) that introduces a known or calculable multiplicativefactor that can be removed through simple techniques such as gainadjustment, if the seed is operated in its linear range of permeability.(More complicated corrections would be required if the seed is apermeable material operated at or near saturation, but it is notenvisioned that such operation would normally be necessary.) Thesubsequent output from this circuitry becomes the functional locatorsignals, sent to analysis in the computer.

The locator signals from the magnetic field sensor, with the aid of thecomputer which has either a lookup table or other means to know thesource magnet field pattern (in conjunction with knowledge of anydeviation of the source magnet from its standard position anddirection), provide a unique direction and magnitude of the oscillatingmagnetic field component of the source magnet, at the position of theprobe, and as measured in the frame of the probe, based on itsorientation. For the front half of the field generating magnet, for agiven current there is a unique pattern of the magnetic field directionand strength. If this pattern is stored in the operating computer, thenknowledge of a measured field at any location near the magnet wouldlocate the point in relationship to the coil. If, in addition, the angleof the axis of the magnet is known to the computer, and the location ofthe coil center is known also, in relation to some fixed referenceframe, then the computer, reading the signals from the implant, canprovide the location and direction of the implant continuously in thatreference frame. If the patient is located relative to that referenceframe, then the implant position and direction is known relative to thepatent. This can be accomplished, for example, by providing fiducialmarkers 242 and 244 on a patient's body and on the magnet and/or magnetarm and locators, as shown in FIG. 2, and by providing a localizer 240to provide a signal representative of the location of these fiducialmarkers to processor 226. Processor 226 can then provide a correction tothe locations sensed —i.e., provide spatial coordinates in thecoordinate system of the room, rather than relative to the magnet 204,based upon the signals from localizer 240. The use of localizers is morefully described in U.S. Provisional Application No. 60/065,107, FiledNov. 12, 1997, and incorporated by reference in its entirety above.

Thus, the invention provides a location and direction of the implantrelative to the magnet, for the purpose of allowing correct orientationof the magnet to guide it in a desired direction, and a location anddirection of the implant relative to the patient, which for somepurposes can be useful to the physician in knowing where in a body ductthe implant is located, without the need for real time imaging apparatusin place if that is not otherwise needed for medical purposes. It canalso be envisioned, however, that some medically useful preoperativeimage of the patient could be put on a computer screen, and some icon ofthe implant superimposed on that image as the implant moves.

In another embodiment of the invention, a further use of the inventionmay be made in which information from instrument module or demodulator224 is used on a display 232 and combined with preoperative images whichmay be supplied to computer 226 in some conventional way. There are manydifferent locations around the front hemisphere of the magnet 204 whichhave the same field line directions, but these locations all differ inthe magnitude of the field. As the field line leaving the magnet in agiven plane, and from a more frontmost point bends, it can have the samedirection that another field line had, in that plane, which left themagnet further back. But it will be further out as it reaches thatangle, and the field will be weaker. This describes the nature of themagnetic field by which there is a unique field line pattern in eachhemisphere of the magnet. (Preferably, magnet 204 will generate ahemispherically symmetrical field. If the magnet is not symmetricend-to-end, there will not be the hemispheric symmetry. Nevertheless,such magnets can be made to function in this invention by merelyadapting the “map” of this field for use in the analysis.)

In the use of this invention, the location and orientation of themagnetometer probe, and hence the implant, is made available to thephysician executing the procedure, and this location is made known usingthe same magnet (or perhaps magnets) as is used to guide the probe. Thislocation can be made known and updated essentially simultaneously withthe actual guidance of the probe. The physician can then know, forexample, the direction of his vision in an endoscope with an opticalfiber system viewing the region ahead of the endoscope.

When location information is displayed and combined with preoperativeimaging, the system must be “calibrated” preoperatively, with somelocalizing method, such as has been described in “Method and ApparatusUsing Shaped Field of Repositionable Magnet to Guide Implant,”incorporated by reference in its entirety above. The display may be ofany type useful for medical imaging. For example, the display could be avolume rendered MRI image, so that the seed can be located in itsphysiological context at each point of the path.

In use, the basic operation of the inventive apparatus is accomplishedby a method comprising the steps of surgically implanting a magneticimplant 100 including an associated magnetic field probe 108 in apatient 200; applying a modulated magnetic field from a calibratedelectromagnet 204; detecting signals from the magnetic probe resultingfrom the application of the modulated magnetic field using demodulator224; and computing the relative location of the magnetic probe, andtherefore, the magnetic implant, with respect to the electromagnet, fromthe detected signals from the magnetic probe. Optionally, the patientand electromagnet may be located with a localizer and an indication ofthe location of the magnetic probe relative to the patient may bedisplayed on a preoperative image of the patient with the indication ofthe location of the magnetic probe superimposed on the display.

Many modifications and variations of the inventive concept would beevident to one skilled in the art upon reading and understanding thisdisclosure, and the embodiments described herein are intended to beexemplary rather than exclusive. For these reasons, the scope of theinvention should be determined with reference to the claims appendedbelow, and the full legal range of equivalents permitted underapplicable law.

1. A magnetic surgical implant comprising a flexible probe having amagnetic seed mounted at a distal end thereof and a magnetic fieldsensor mounted near the magnetic seed and in a fixed relationshipthereto so that as an external magnetic field acts on said magnetic seedto guide or propel it through a patient's body, the magnetic fieldsensor is also acted upon and provides an output which can be correlatedwith the position of the magnetic seed.
 2. The magnetic surgical implantof claim 1 wherein the magnetic field sensor comprises a 3-axismagnetometer probe.
 3. The magnetic surgical implant of claim 2 whereinthe magnetic field sensor is mounted within the flexible probe.
 4. Themagnetic surgical implant of claim 3 further comprising at least onesensor conductor connected to the magnetic field sensor and extendingthrough the flexible probe for carrying signals from the magnetic fieldsensor to a receiver.
 5. The magnetic surgical implant of claim 4wherein the flexible probe is an endoscope.
 6. The magnetic surgicalimplant of claim 4 wherein the flexible probe is a catheter.
 7. Themagnetic surgical implant of claim 4 wherein the magnetic seed is alsomounted within the flexible probe and at the tip thereof.
 8. Themagnetic surgical implant of claim 4 wherein the sensor conductorcomprises at least one electrical conductor.
 9. A device for providingposition data for a magnetic surgical implant having a magnetic fieldsensor which is guided or propelled through a patient's body by anexternal magnet, said device comprising a current source for supplying amodulated electrical current to the external magnet to thereby generatean oscillating magnetic field component, a demodulator coupled to themagnetic field sensor and responsive to a magnetic field direction andmagnitude of the oscillating magnetic field component to provide asignal indicative of the oscillating magnetic field component at thelocation of the magnetic field sensor, a memory containing arepresentation of a relationship of spatial locations to a magneticfield pattern produced by the external magnet, and a data processorcoupled to the demodulator and the memory and configured to compute aspatial location of the magnetic field sensor as a function of thesignal indicative of the oscillating magnetic field component.
 10. Thedevice of claim 9, wherein the representation of the relationship ofspatial locations to the magnetic field pattern produced by theelectromagnet is a representation in a reference frame, theelectromagnet is moveable, and the device further comprises a localizerconfigured to provide a signal representative of a position andorientation of the electromagnet to the processor, and the processor isconfigured to apply a correction based upon the position and orientationprovided by the locator.
 11. The device of claim 10 wherein thelocalizer comprises a sensor mounted on the electromagnet and a firstplurality of fiducial markers positioned on the patient and a secondplurality of fiducial markers positioned on the electromagnet.
 12. Thedevice of claim 9 and further comprising a display coupled to theprocessor, the processor further being configured to display on saiddisplay an image representing the spatial location of the magnetic fieldsensor and a preoperative image of a patient in which the magnetic fieldsensor is implanted, the location image and the preoperative image beingsuperimposed on one another.
 13. The device of claim 9, in which theelectromagnet is configured to generate a hemispherically symmetricalfield.
 14. The device of claim 9, in which the electromagnet isconfigured to generate a hemispherically asymmetric field.
 15. Thedevice of claim 9 in which the electromagnet comprises a plurality ofseparate coils.
 16. A method for locating a magnetic surgical implant,comprising the steps of: (a) surgically implanting a magnetic implantincluding an associated magnetic field probe in a patient; (b) applyinga modulated magnetic field from an external electromagnet; (c) detectingsignals from the magnetic probe resulting from the application of themodulated magnetic field; and (d) computing the relative location of themagnetic probe, and therefore, the magnetic implant, with respect to theelectromagnet, from the detected signals from the magnetic probe. 17.The method of claim 16, and further comprising the steps of locating thepatient and electromagnet with a localizer and displaying an indicationof the location of the magnetic probe relative to the patient on adisplay.
 18. The method of claim 17, and further comprising the step ofsuperimposing a preoperative image of the patient on the display withthe indication of the location of the magnetic probe.
 19. The method ofclaim 18 wherein the step of applying a modulated magnetic field from anexternal electromagnet comprises the steps of selecting a currentrequired by the external electromagnet to guide the magnetic implant andmodulating the selected current with a modulating signal; and furthercomprising the step of guiding the magnetic probe with the modulatedmagnetic field.
 20. The method of claim 16 wherein the step of applyinga modulated magnetic field from an external electromagnet comprises thesteps of selecting a current required by the external electromagnet toguide the magnetic implant 5 and modulating the selected current with amodulating signal; and further comprising the step of guiding themagnetic probe with the modulated magnetic field.